Telomerase is inactive in most somatic cells, but has been found to be reactivated in a majority of cancers. Our principal goal was to test whether the presence of telomerase activity concurred with positive cytology, and was thus of potential use in detecting cancer cells in effusions. The telomeric repeat amplification protocol (TRAP) assay and cytological examination were performed in a blinded fashion on 91 unselected effusions, for which laboratory processing was done according to standard procedures. In our series, 30% (27/91) of samples were found to be malignant by cytology. Of these, 19 (70%) were also positive in the TRAP assay. Of the 8 telomerase-negative cytology-positive samples, RNA integrity was generally poor, indicating suboptimal sample conservation for molecular analysis. Negative cytology in the presence of telomerase activity was observed in 17 effusions. Of these, 11 were from patients with advanced cancer, and thus a diagnosis of malignant effusion should be suspected. The TRAP assay for telomerase activity holds promise in the analysis of effusions, but its routine use as an adjunct to cytology awaits further confirmation of its positive predictive value